Masking in Premiere Pro

I am trying to add a mask on a clip. But, no matter what tool I use (rectangle, ellipse, or pen) all that happens is the mask outline shows. The mask does not apply. I have watched many videos and on every one, the working area goes black except for the masked area. Inverting does nothing. The blue shape just stays there with no effect. When I click on another clip, the masked line disappears. Any thoughts or suggestions?